Hall added that even after the first wave of the vaccine is made available, manufacturers will continue to churn out 20 million doses each week.

"Early on, there were issues with production yield, but that has improved significantly over the past several weeks," Hall said. He noted that other confounding factors, such as one company finishing production of its regular seasonal flu vaccine behind schedule, further slowed the production of the swine flu vaccine.

Still, Monday's revelation comes amid warnings from infectious disease experts of a fall resurgence of the pandemic strain of the H1N1 virus. And Hall said the results of the tests that will determine whether one or two doses of the vaccine are necessary to prevent sickness have not yet yielded results.

Dr. William Schaffner, chairman of the Department of Preventive Medicine at Vanderbilt University in Nashville, Tenn., said that the latest status report is troubling news.

"We've been in a race to try and get this vaccine manufactured," he told ABC News' Diane Sawyer on "Good morning America" on Tuesday. "It makes us all a little bit nervous.

"We won't have as much vaccine to start a vaccine program, and we're worried that we will have people sick that could have been prevented and people in the hospital that could have been prevented."

Swine Flu Vaccine Production Met Delays, Other Hurdles

Disease specialists have pointed out that despite these numbers the impact of the swine flu has been relatively mild. And Hall said that the anticipated shortfall in the vaccine supply was not completely unanticipated – a statement with which influenza experts agreed.

"A lot of things go into the making of a flu vaccine, which is a complex, multi-step process," said Dr. John Treanor, professor of Medicine, Microbiology and Immunology at the University of Rochester Medical Center in Rochester, N.Y. " I actually think it is amazing that 45 million doses will be ready by October; after all, the virus wasn't even recognized until the end of April."